The qualitative determination of the pH value of a medium is an essential process parameter in many processes. Various techniques for determining the pH value are therefore known from the prior art.
Typically, ion-selective electrodes are used to measure ion activity. The most common variant is apparently a glass electrode. Glass electrodes are normally understood to be solid-state membranes consisting of siliceous glasses, which are usually melted from oxides or carbonates and then changed to the final version by glassblowing. If such a glass electrode is dipped into an aqueous solution, a hydrated film forms on the pH value-sensitive membrane glass. This is also done on the inner side of the glass membrane, which is in contact with a specific buffer solution such as a potassium chloride buffer (PCL). A reference electrode is also needed in addition to the glass electrode, so that the pH value can be measured.
The design of such glass electrodes is generally very large. For this reason, the possible integration density, i.e., the number of other sensors per unit area, is very restricted, and is therefore also not suitable for use in processes with very small amounts of the analytes, such as in the nL, μL, or mL range. Furthermore, the problem with such glass electrodes is that they may not be used in many applications, due to their fragility and the possibility of contamination in the event that the glass breaks.
From the prior art, ion-selective field effect transistors (ISFETs) are known that measure the pH value with a pH-sensitive film (such as Ta2O5, SiN, and Al2O3) which is applied to the gate. With such ISFETs, the potential to be measured is converted to a current signal.
The disadvantage of ISFETs is that they react very sensitively to radiation. Particularly when used in biological processes, sterilization is essential, and recourse is frequently made to gamma sterilization. This is not feasible with ISFET sensors. Furthermore, ISFETS are generally produced in CMOS (complementary metal-oxide semicoductor) processes, since the electronics unit for evaluation is applied directly to the chip. This limits a potential integration of other measured variables, since significant incompatibilities exist between the processes. Accordingly, at high temperatures, many metals cannot be used at all, or only sparingly.
Likewise, thick-film electrodes based upon RuO2 or other metal oxides with a pH value sensitivity are known from the prior art, which typically work by means of a metallic fixed lead.
The disadvantage of thick-film electrodes is generally the strong dependence of the measuring signal upon the redox potential of the analyte. For this reason, such thick-film electrodes are of interest only in areas of use where the redox potential of the analyte remains basically constant.
Likewise, thick-film sensors are known from the prior art. These work on the basis of a pH-sensitive glass that is measured by means of a fixed lead (made of platinum, for example). High-temperature co-fired ceramics (HTCC for short) are used as a substrate for the thick-film sensors.
The use of HTCC processes limits the possible materials and structured precision, due to the shrinkage during the HTCC process, as well as the compatibilities of the materials with each other. Due to the reproducibility of the shrinkage while baking, these sensors are kept relatively large. Furthermore, all materials involved must withstand very high temperatures.
In addition to a high-temperature, multilayer ceramic as a substrate, steel substrates are also used. The use of steel substrates is associated with the disadvantage that they either oxidize while baking the thick-film paste, which can cause contamination of the sensitive layers, or baking must occur in a low oxygen atmosphere, which, in turn, cannot be done with many materials, or is incompatible with them.
Furthermore, dye sensors are known from the prior art that change their color or fluorescence times, depending upon the pH value. These are read out by an optical system.
The disadvantage in this context is that the dye sensors must be read out by very complex optical systems. In addition, they frequently have low long-term stability. Furthermore, they frequently require recalibration after sterilization processes. Since the quality of the connection of the optical system plays a role in the measuring precision, frequently, at least a one-point calibration must be performed in disposable applications (i.e., single use products). Another disadvantage that such dye sensors have is a relatively low long-term stability. In the final analysis, this makes the sensor expensive to use. Furthermore, dye sensors degrade very quickly—for example, by bleaching.